In my research, I investigate the impact of microbial activity on the corrosion behavior of ductile iron casting, specifically focusing on Escherichia coli (E. coli), a common bacterium found in reclaimed water systems. Corrosion is a significant factor leading to material failure, with microbial-influenced corrosion accounting for approximately 20% of all corrosion-related losses. Ductile iron casting, such as QT500-7 grade, is widely used in water distribution pipelines due to its cost-effectiveness and mechanical strength. However, in environments like reclaimed water, which contains organic nutrients that promote bacterial growth, ductile iron casting is susceptible to accelerated degradation. This study aims to elucidate how E. coli influences the corrosion processes of ductile iron casting through weight loss measurements, electrochemical analyses, and surface characterization, providing insights for corrosion mitigation in infrastructure.
The importance of this work stems from the increasing use of reclaimed water as a freshwater resource, where bacterial colonization on metal surfaces can lead to biofilm formation and localized corrosion. E. coli is a gram-negative bacterium that thrives in such conditions, and its metabolic activities can alter the local electrochemical environment at the metal-solution interface. While prior studies have examined E. coli effects on metals like stainless steel, limited research exists on its interaction with ductile iron casting. By employing a combination of techniques, I assess corrosion rates, electrochemical parameters, and morphological changes to understand the underlying mechanisms. This comprehensive approach allows for a detailed evaluation of how microbial presence exacerbates corrosion in ductile iron casting, which is critical for developing protective strategies in water systems.
In this study, I use QT500-7 ductile iron casting as the primary material, with its chemical composition detailed in Table 1. The ductile iron casting samples are prepared for various tests, including weight loss coupons, electrochemical electrodes, and surface analysis specimens. All samples are sterilized under UV light before experimentation to ensure controlled conditions. The E. coli strain is cultured in a liquid medium (CM0002) containing peptone, beef extract, and sodium chloride, adjusted to pH 7.0, to simulate nutrient-rich reclaimed water. The bacterial inoculation is performed under aseptic conditions, and cultures are maintained at 37°C in a shaking incubator to promote growth. This setup mimics the environmental conditions where ductile iron casting pipelines are exposed to microbial activity.
| Element | C | Si | Mn | P | S | Fe |
|---|---|---|---|---|---|---|
| Content | 3.42 | 2.74 | 0.29 | 0.032 | 0.014 | Balance |
For weight loss corrosion tests, I immerse ductile iron casting coupons in both sterile and E. coli-inoculated media for periods of 5, 10, 15, and 20 days. The corrosion rate is calculated using the formula:
$$ V = \frac{\Delta W}{S \cdot t} $$
where \( V \) is the corrosion rate in g·m⁻²·h⁻¹, \( \Delta W \) is the mass loss in grams, \( S \) is the surface area in m², and \( t \) is the exposure time in hours. This provides a quantitative measure of how E. coli affects the degradation of ductile iron casting over time. Electrochemical tests are conducted using a three-electrode system with the ductile iron casting as the working electrode, a saturated calomel reference electrode, and a platinum counter electrode. I measure open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization to assess corrosion tendencies and kinetics. The EIS data are fitted with equivalent circuits to extract parameters like charge transfer resistance (\( R_{ct} \)) and biofilm capacitance (\( Q_{bf} \)), which reflect the influence of microbial activity on ductile iron casting corrosion.
Surface characterization involves scanning electron microscopy (SEM) to observe corrosion morphology and bacterial distribution on ductile iron casting samples. After exposure, samples are fixed with glutaraldehyde, dehydrated, and examined to visualize biofilm formation and pitting. Additionally, I collect corrosion products for energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis to determine elemental composition and phase identification. This multi-faceted methodology enables a holistic understanding of the corrosion behavior of ductile iron casting in the presence of E. coli, bridging macroscopic weight loss with microscopic electrochemical and surface changes.
The weight loss results reveal a significant acceleration of corrosion in ductile iron casting due to E. coli. As shown in Table 2, the corrosion rate in the E. coli-inoculated system (\( V_E \)) increases over time, surpassing that in the sterile system (\( V_0 \)). The ratio \( V_E / V_0 \) grows from 1.03 at 5 days to 4.21 at 20 days, indicating that microbial activity progressively enhances the corrosion of ductile iron casting. This trend suggests that E. coli colonization leads to biofilm development, which entraps metabolic byproducts and creates localized microenvironments that aggravate corrosion. The ductile iron casting surface becomes more susceptible to attack as bacteria proliferate, aligning with observations in other microbial corrosion studies where biofilms accelerate metal degradation through chemical and electrochemical interactions.
| Time (days) | \( V_0 \) (g·m⁻²·h⁻¹) | \( V_E \) (g·m⁻²·h⁻¹) | \( V_E / V_0 \) |
|---|---|---|---|
| 5 | 0.0102 | 0.0105 | 1.03 |
| 10 | 0.0135 | 0.0218 | 1.32 |
| 15 | 0.0181 | 0.0335 | 1.85 |
| 20 | 0.0162 | 0.0682 | 4.21 |
Electrochemical measurements further elucidate the corrosion dynamics of ductile iron casting under microbial influence. The open-circuit potential (OCP) trends, depicted in Figure 1 (though not shown here, described textually), indicate that in the sterile system, OCP initially shifts positively due to corrosion product film formation, then negatively as the film deteriorates. In contrast, the E. coli system shows a continuous negative shift in OCP over 8 days, reflecting increased corrosion tendency as bacteria grow and form biofilms on the ductile iron casting surface. This negative shift is consistent with enhanced anodic dissolution facilitated by microbial metabolic processes, which lower the local pH and increase oxygen concentration gradients, driving corrosion reactions.
Potentiodynamic polarization curves after 8 days of immersion demonstrate that the E. coli system has a more negative corrosion potential (\( E_{corr} \)) and a higher corrosion current density (\( i_{corr} \)) compared to the sterile system. Using Tafel extrapolation, I derive the electrochemical parameters summarized in Table 3. The corrosion current density in the E. coli system is approximately six times higher than in the sterile system, confirming that microbial activity significantly accelerates the corrosion rate of ductile iron casting. The Tafel slopes \( \beta_a \) and \( \beta_c \) also change, indicating alterations in anodic and cathodic reaction mechanisms due to bacterial presence. This electrochemical behavior underscores how E. coli promotes corrosion by facilitating charge transfer processes at the ductile iron casting interface, likely through the secretion of extracellular polymeric substances that complex with iron ions and enhance dissolution.
| System | \( i_{corr} \) (A·cm⁻²) | \( E_{corr} \) (V vs. SCE) | \( \beta_a \) (V·decade⁻¹) | \( \beta_c \) (V·decade⁻¹) |
|---|---|---|---|---|
| Sterile | 2.5008 × 10⁻⁶ | -0.85 | 0.154 | 0.087 |
| E. coli | 1.4103 × 10⁻⁵ | -0.93 | 0.355 | 0.121 |
Electrochemical impedance spectroscopy (EIS) provides insights into the interfacial processes on ductile iron casting. Nyquist plots show that in the sterile system, the semicircle radius increases over time, suggesting the formation of a protective corrosion product layer. However, in the E. coli system, the radius initially increases but then decreases after 5 days, indicating biofilm instability and enhanced corrosion activity. I fit the EIS data using equivalent circuits; for the sterile system, a simple Randles circuit with solution resistance (\( R_s \)), double-layer capacitance (\( Q_{dl} \)), and charge transfer resistance (\( R_{ct} \)) suffices, while for the E. coli system, additional elements for biofilm resistance (\( R_{bf} \)) and capacitance (\( Q_{bf} \)) are included. The fitted parameters, presented in Table 4, reveal that \( R_{ct} \) in the E. coli system is about one-tenth of that in the sterile system after 3 days, highlighting reduced charge transfer resistance and faster corrosion kinetics due to microbial action on ductile iron casting.
| Time (days) | System | \( R_s \) (Ω·cm²) | \( R_{ct} \) (Ω·cm²) | \( Q_{dl} \) (μF·cm⁻²) | \( R_{bf} \) (Ω·cm²) | \( Q_{bf} \) (μF·cm⁻²) |
|---|---|---|---|---|---|---|
| 1 | Sterile | 11.78 | 4704 | 7.481 × 10⁻⁵ | — | — |
| 1 | E. coli | 13.78 | 6692 | 1.257 × 10⁻⁵ | 4295 | 0.019 × 10⁻⁵ |
| 2 | Sterile | 10.95 | 5153 | 4.592 × 10⁻⁵ | — | — |
| 2 | E. coli | 12.17 | 6774 | 1.203 × 10⁻⁵ | 5411 | 0.308 × 10⁻⁵ |
| 3 | Sterile | 9.97 | 6828 | 2.467 × 10⁻⁵ | — | — |
| 3 | E. coli | 11.70 | 2496 | 1.171 × 10⁻⁵ | 13504 | 225.200 × 10⁻⁵ |
| 4 | Sterile | 9.39 | 9586 | 1.823 × 10⁻⁵ | — | — |
| 4 | E. coli | 11.28 | 1485 | 1.204 × 10⁻⁵ | 17576 | 0.174 × 10⁻⁵ |
| 5 | Sterile | 8.67 | 12173 | 1.614 × 10⁻⁵ | — | — |
| 5 | E. coli | 11.19 | 1273 | 1.229 × 10⁻⁵ | 20073 | 40.010 × 10⁻⁵ |
| 6 | Sterile | 8.70 | 14327 | 2.007 × 10⁻⁵ | — | — |
| 6 | E. coli | 11.64 | 1362 | 1.265 × 10⁻⁵ | 17781 | 7.290 × 10⁻⁵ |
| 7 | Sterile | 8.63 | 16899 | 2.226 × 10⁻⁵ | — | — |
| 7 | E. coli | 11.05 | 1463 | 1.294 × 10⁻⁵ | 17140 | 7.590 × 10⁻⁵ |
| 8 | Sterile | 8.68 | 18521 | 2.426 × 10⁻⁵ | — | — |
| 8 | E. coli | 11.23 | 1513 | 1.358 × 10⁻⁵ | 14184 | 0.123 × 10⁻⁵ |
The surface morphology of ductile iron casting after corrosion exposure provides visual evidence of microbial influence. SEM images show that in the sterile system, the ductile iron casting surface exhibits only minor uniform corrosion, with graphite nodules visible as dark circular features. In contrast, the E. coli system displays numerous corrosion pits after 4 days, which expand and coalesce into broader attacked areas by 8 days, indicating pitting corrosion that progresses towards general corrosion. This localized attack is characteristic of microbial corrosion, where biofilm heterogeneity creates oxygen concentration cells and acidic microenvironments that drive anodic dissolution at specific sites on the ductile iron casting. The presence of E. coli cells, observed as rod-shaped structures on the surface, confirms bacterial adhesion and biofilm formation, which exacerbate the corrosion process by physically shielding areas and promoting differential aeration.
Energy-dispersive X-ray spectroscopy (EDS) analysis of corrosion products on ductile iron casting reveals higher iron (Fe) and oxygen (O) content in the E. coli system compared to the sterile system, as summarized in Table 5. This suggests that microbial activity accelerates the oxidation of iron, leading to more extensive corrosion product formation. The atomic percentages indicate that E. coli enhances the conversion of metallic iron to iron oxides and hydroxides, which are key components of the corrosion layer on ductile iron casting. This aligns with the weight loss and electrochemical data, where increased corrosion rates correlate with greater iron release and oxidation in the presence of bacteria.
| System | O | Fe | C | Others |
|---|---|---|---|---|
| Sterile | 34.62 | 25.17 | 16.56 | 23.65 |
| E. coli | 44.07 | 30.18 | 18.44 | 7.31 |
X-ray diffraction (XRD) patterns of corrosion products from ductile iron casting in both systems identify Fe(OH)₂ and α-FeOOH (goethite) as the primary phases. The diffraction peaks are more intense in the E. coli system, reflecting a higher quantity of corrosion products due to accelerated corrosion. This indicates that E. coli does not alter the fundamental corrosion pathway of ductile iron casting but rather speeds up the reaction kinetics. The formation of these iron hydroxides and oxyhydroxides is consistent with aerobic corrosion processes, where dissolved oxygen reacts with iron to form layered deposits. The microbial biofilm likely facilitates this by increasing local oxygen reduction rates or by producing metabolites that complex with iron ions, as described by the following generalized reactions:
Anodic reaction: $$ \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- $$
Cathodic reaction: $$ \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- $$
Overall: $$ 2\text{Fe} + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{Fe(OH)}_2 $$
Further oxidation: $$ 4\text{Fe(OH)}_2 + \text{O}_2 \rightarrow 4\alpha\text{-FeOOH} + 2\text{H}_2\text{O} $$
In the context of ductile iron casting, the graphite nodules act as cathodic sites, accelerating the anodic dissolution of the iron matrix, especially under microbial influence. E. coli biofilm formation enhances this galvanic coupling by creating localized acidic conditions through metabolic acid production, which can be represented by:
$$ \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} $$
$$ \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^- $$
The decrease in local pH increases the solubility of iron oxides, promoting further corrosion of ductile iron casting. Additionally, extracellular polymeric substances (EPS) secreted by E. coli can chelate iron ions, forming complexes that enhance iron dissolution rates. This chelation effect can be modeled using stability constants, where the equilibrium between free and complexed iron influences corrosion dynamics:
$$ \text{Fe}^{2+} + \text{EPS} \rightleftharpoons \text{Fe-EPS complex} $$
with stability constant $$ K = \frac{[\text{Fe-EPS}]}{[\text{Fe}^{2+}][\text{EPS}]} $$
The electrochemical impedance data support this, as the low \( R_{ct} \) values in the E. coli system indicate facilitated charge transfer due to such complexation and biofilm porosity. The biofilm capacitance \( Q_{bf} \) variations over time reflect changes in biofilm thickness and morphology on ductile iron casting, which directly impact corrosion rates. For instance, initial biofilm growth increases \( Q_{bf} \), but as the biofilm matures and becomes more porous, it may lead to fluctuations, as seen in Table 4.
To quantify the corrosion acceleration due to E. coli, I derive a corrosion rate enhancement factor \( \alpha \) defined as:
$$ \alpha = \frac{V_E}{V_0} = \frac{i_{corr,E}}{i_{corr,0}} \cdot \frac{A_E}{A_0} $$
where \( i_{corr} \) values are from polarization curves, and \( A \) represents the effective corroding area. Given the pitting morphology, \( A_E > A_0 \), contributing to higher \( \alpha \) values over time. This factor highlights how microbial activity not only increases current density but also expands the active corrosion area on ductile iron casting. From the data, \( \alpha \) ranges from 1.03 at 5 days to 4.21 at 20 days, demonstrating a time-dependent synergistic effect between E. coli and ductile iron casting corrosion.
The implications of these findings for practical applications are significant. In reclaimed water pipelines made of ductile iron casting, E. coli colonization can lead to premature failure through pitting and localized corrosion. Preventive measures, such as biocidal treatments or corrosion-resistant coatings, should consider the dynamic nature of microbial biofilms. For example, based on the corrosion rate data, the service life of ductile iron casting components in E. coli-contaminated environments could be reduced by a factor proportional to \( \alpha \). If we assume a baseline corrosion rate \( V_0 \) for sterile conditions, the time to failure \( t_f \) under microbial influence can be estimated as:
$$ t_f = \frac{\delta}{\alpha \cdot V_0} $$
where \( \delta \) is the critical corrosion depth. For ductile iron casting with typical wall thickness, this underscores the need for regular monitoring and maintenance in systems exposed to bacterial loads.
In conclusion, my investigation demonstrates that Escherichia coli substantially accelerates the corrosion of ductile iron casting, such as QT500-7, in reclaimed water-simulating conditions. Weight loss measurements show increasing corrosion rates over time, with the E. coli system exhibiting rates up to four times higher than sterile systems. Electrochemical tests reveal lower corrosion potentials, higher current densities, and reduced charge transfer resistances in the presence of E. coli, indicating enhanced anodic dissolution and altered reaction kinetics. Surface analyses confirm pitting corrosion and greater iron oxide formation, with corrosion products consisting mainly of Fe(OH)₂ and α-FeOOH, unchanged in composition but increased in quantity. These findings emphasize that microbial activity, through biofilm formation and metabolic processes, exacerbates corrosion in ductile iron casting without altering the fundamental corrosion pathway. This work provides a foundation for developing targeted anti-corrosion strategies for ductile iron casting infrastructure in microbial-rich environments, highlighting the importance of integrating microbiological factors into corrosion management practices. Future studies could explore synergistic effects with other bacteria or evaluate protective coatings to mitigate microbial corrosion in ductile iron casting systems.